Phenylpropanoids constitute an important and ubiquitous class of molecules found in fungi, bacteria, plants, and mammals. Phenylpropanoids are used by plants for many diverse functions, such as nodulation gene inducers and stress response molecules. The O-methytransferases (OMTs) catalyze a number of methylation reactions in the phenylpropanoid metabolic pathway. Caffeic acid 3-O-methyltransferase (COMT) and caffeoyl CoA 3-O-methyltransferase (CCOMT) both catalyze the methylation of caffeic and ferrulic acid, a critical step in lignin synthesis. Isoflavone O-methyltransferase (IOMT) converts daidzein to formononetin, a precursor to the phytoalexin medicarpin. Isoliquiritigenin 2'-O-methyltransferase (ISOMT) methylates 2',4,4'-trihydroxychalcone to form 4,4'-dihydroxy-2'-methoxychalcone, a powerful nodulation gene inducer. Crystallization of these enzymes has been successful and all diffract to varying degrees. In order to fully understand the mechanistic details of these reactions, high resolution data is critical.